This invention relates to apparatus and a method for drying elongated webs, especially freshly printed webs. More particulaly, this invention relates to apparatus and a method which are useful for the drying of webs of polymeric material, such as vinyl, to which chemical or water-based inks have been applied.
In general, in conventional apparatus and methods the drying of webs is accomplished by a mechanical removal and evaporation of residual liquids. As is well known, liquids can be made to evaporate by causing their vapor pressure to exceed ambient pressure. This is most readily achieved by raising the temperature of the liquid sought to be evaporated, and explains why conventional drying apparatus, such as the apparatus disclosed in U.S. Pat. No. 4,125,948, issued Nov. 21, 1978, to Henry H. Hering, Jr. et al, suggests the technique of blowing heated air onto the web to be dried. But the use of impinging hot air in the conventional manner dictates certain compromises, the most serious of which being that the temperature of the air must be carefully monitored and controlled if damage to the web is to be avoided. Drying by the impingement of hot air is further complicated by the unavoidable existence of a dense boundary layer of air at the surface of the web. The boundary layer of air at the surface of the web. The boundary layer, being typically approximately 1.1 to 1.3 time as dense as the surrounding air, is heat absorbtive, and moreover, heavily charged with staic electricity, which makes it highly cohesive. Both its density and its cohesiveness make the boundary layer an effective insulator of the web and a barrier to the introduction of heat for evaporation. Thus, conventional drying apparatus and methods, using impingement, are crude in terms of their application of physical principles, and wasteful of energy.